This invention relates to garage shifting of a motor vehicle automatic transmission and, more particularly, to a shift control method that minimizes output torque disturbances due to garage shifting.
In general, a motor vehicle automatic transmission includes a number of gear elements and hydraulically-controlled friction elements (referred to herein as clutches) that are selectively engaged or disengaged according to a predetermined schedule for establishing a desired gear ratio between the transmission input and output shafts. In gear ratio shifting, various hydraulic and/or electro-hydraulic mechanisms are designed to control the clutch flow and pressure so that the shift is substantially imperceptible to the vehicle occupants. This is particularly important in the case of range or garage shifts since the gear element lash can be significant (especially in all-wheel-drive vehicles), and an objectionable driveline torque disturbance can occur if the gear lash is taken up too quickly. Various hydraulic flow control mechanisms can be employed to reduce fluid flow during garage shifting, but such mechanisms can add considerable cost to the transmission, and introduce durability concerns under cold temperature and high input torque conditions. Accordingly, what is needed is a cost-effective way of minimizing garage shift driveline torque disturbance without compromising the durability of the transmission friction elements.
The present invention is directed to an improved control method for a motor vehicle automatic transmission that minimizes output torque disturbances due to garage shifting. Under certain predefined conditions, garage shifts to the forward (Drive) range are carried out by initially commanding a shift to an upper forward gear ratio having a relatively low torque advantage to smoothly condition the transmission gearset for forward operation, and then commanding a shift from the upper forward gear ratio to a lower forward ratio that is ordinarily used to launch the vehicle. The shift from the upper gear ratio to the lower gear ratio is initiated when the initial shift is substantially complete or when a predetermined period of time has elapsed since shift initiation. Engine management controls are also used to reduce the transmission input torque during garage shifts to both forward and reverse ranges.